Once in a while you get a mean one, which you sound like you have, but most of the time they are pretty docile. Not to undermine your advice, but 3 is not really a good number of CA cichlids to divert aggression (smallest one ends up getting picked on) you usually need more like 5-6 for it to really work. And 5-6 JDs would be far too many in there with an Oscar. I would still recommend trying one JD and see how it works out.
As far as the size, O's grow much quicker than JDs, but as long as the JD can't fit in it's mouth it should be fine.
And as for the planted tank idea, that is a nice setup. Any schoolers which are fast enough to avoid the GT would be fine, although you kinda have to go into it willing to lose a couple. Bristlenose plecos work well as bottom dwellers with cichlids like this.